The Maps API provides a node.js based API that allows you to generate maps based on data hosted in your CartoDB account by applying custom SQL and CartoCSS to the data
Like the other components of CartoDB is Open Source and you can find the source code at CartoDB/Windshaft-cartodb
You can find usage documentation at https://docs.carto.com/cartodb-platform/maps-api.html
Although you can chechout any branch of the repository most of them are usually work in progress that is not guaranteed to work. In order to run a production ready Maps API service you need to use master branch.